Black--the color defined by the absence of light (which is part of why some people say black is not really a color).
This post was edited by Benedict Arnold at December 20, 2017 1:23 PM MST
Radiation is emitted from the event horizon of the hole whenever some piece of matter fall into it, But the radiation wavelengths is far outside of the (fairly harmless) band that we ascribe colors to.
There is an 11 second video taken over 15 years. It is the center of our galaxy and shows a handful of stars orbiting nothing visible. I couldn't find it on youtube, though.
Black holes are the most dense objects in the universe. Because of their density, they form extremely powerful gravitational fields. Black holes absorb all surrounding matter and energy within a certain proximity. For this reason, these celestial objects emit no light and therefore do not have a color. Astronomers can detect them, however, by monitoring the properties of the materials and energy surrounding them.
An infinitely dense region of space, also known as a singularity, has no color that we know of.
This post was edited by carbonproduct at December 20, 2017 1:22 PM MST
Massive black holes can't be seen, but scientists can see the stars that rotate around it and can tell how fast they are moving. See photo from NASA. 
Benedict Arnold 